Round ringless mold and triangular spoke sprue

ABSTRACT

A mold assembly and sprue configuration for dental investment casting includes a ring shaped casing, a base and a cover all made of resilient plastic material and fictionally engaged with each other. The base includes a raised neck having a top surface with a sighting bead for use in mounting wax forms on the sprue, positioned within the casing. The sprue includes a post for engaging a blind bore in a hub of the base. The cover includes a downwardly extending plug which extends into the body of the casing, and a curved lip with recess for facilitating the pouring of investment solution into the mold. The sprue has three spokes and a ring which are all triangular in cross-section to reduce the amount of metal needed in the precision casting to be made with the invention and also to provide a convenient flat upper surface for mounting wax forms to the sprue.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates, in general, to plastic molds or castingsfor making investment molds and, more particularly, to a new and usefulround ringless mold and spoked sprue for making investment molds forprecision casting of dental prostheses.

In the dental field, it is common to use lost wax methods for producingdental castings. Lost wax methods utilize investment solution which isat least partly cured in a vessel in order to produce a dental casting.

In many known casting devices, a metal or plastic ring is force fit intoa groove in a base to form the mold or vessel for investing theinvestment solution.

The use of polyvinyl chloride (PVC) or other elastomer, in theconstruction of both the base and ring is desirable, among otherreasons, because such materials can accommodate expansion of theinvestment casting as it cures and such materials are more readilycleaned after the hardened investment is removed.

Molds and sprues in this field are illustrated in U.S. Pat. Nos.4,573,921; Des. 329,900; 5,406,999 and 5,469,908.

Various patents in this field disclose various sprue plus moldstructures. See for example U.S. Pat. Nos. 1,458,835; 1,939,479;2,274,186; 3,648,760; 3,939,898 and 4,558,841.

U.S. Pat. No. 4,962,909 discloses a mold for dentistry which includes alower funnel shaper for shaping the funnel portion of an investment, andan upper blind funnel for displacing a core portion of the investmentfrom within a pattern of dental forms, to reduce distortion in the formswhich would have been caused by an expansion of the investment as itcured.

U.S. Pat. No. 5,183,095 discloses a mold for precision casting made ofthermoplastic material.

The advantage of using thermoplastic material for a mold in the field ofdental casting has been described in Materials and Materials Science,No. 3 Report 303, March 1981, "The Equi-Spansion Technique for CastingsWithout Metal Rings" by Engelmann and Blechner. The advantages ofmaintaining uniform investment wall thickness around a wax form set byusing an oval mold for making a precision casting has been disclosed inTrends & Techniques in the Contemporary Dental Laboratory, Volume 8, No.3, "The Effects of Shape and Size of Investment Heating and CoolingRates", by Berger and Benson.

A wax sprue for dental casting is also currently being marketed byAmerican Diversified Dental Systems, known as The Spiroloop. Thisproduct comprises a central post or hub, meant to be attached to thebase of a mold. Three spokes extend outwardly from the hub and arecurved helically. A ring connects the outer ends of the spokes and formsa support for wax forms to be attached to the top of the sprue. Thespokes and ring all have circular cross-sections. Accordingly, after thelost wax process is completed, the metal casting includes thickcylindrical and helical spoke segments and a thick ring with cylindricalcross-section. This represents the use of a large quantity of metal. Theadvantages of the spiral wound spokes are said to include a more quickand smooth flow of metal without the creation of restrictions that causeturbulence.

A need remains for an improved mold with sprue that reduces distortionof the wax forms in the investment casting, reduces the amount of wastedcasting investment and reduces the amount of excess metal in the spruepattern of the precision casting.

SUMMARY OF THE INVENTION

The present invention comprises an improved mold and sprue for theproduction of dental castings using a lost wax method.

The mold of the present invention receives a liquid investment solutionwhich cures to produce an investment casting. The invention comprising around casing having a collar at an inner surface of a lower end of thecasing. The casing is fictionally engaged with a base having a roundneck which extends upwardly, in an axial direction or at a slightincline for fictionally engaging the collar of the casing. A sprue withspokes and a ring, or more correctly, a sprue former, carries aplurality of wax forms on the ring and is located on the base forproducing the metal precision dental casting.

Both the casing and the base are made of a resilient plastic materialsuch as PVC. The neck of the base is in the form of a ring with aconical or planar upper ring shaped surface. A raised circular bead isformed concentrically on the ring shaped surface, between inner andouter axial faces of the neck.

The bead is used as a sighting tool for the lab technician to use forplacement of the wax forms on the ring. All wax forms must be placedinside the sighting bead in order to insure sufficient investmentmaterial thickness in the investment casting, around the wax forms.

The base further comprises a conical hub at a center of the base whichsupports the wax or plastic sprue or sprue former. A ring shaped grooveextends between the inside face of the neck, and the hub. A blindcylindrical hole is provided at the top center of the hub for receivinga post at the bottom of the spoked sprue. The post of the sprue isadvantageously tapered with a large diameter bottom and a smallerdiameter top. This helps firmly fix the post into the hole of the hub.The ring shaped groove around the hub as well as the conical shape ofthe hub form the funnel portion of the investment casting while reducingthe amount of wasted investment material in the funnel area of theinvestment casting.

A cover, which, like the casing and base, is also made of elastomer suchas PVC, is used to cover the open upper end of the casing. The coverincludes an upstanding lip and peripheral opening for receiving liquidinvestment material, similar to the lip and opening disclosed in U.S.Pat. No. 5,469,908. Depending from the center of the cover is a blindfunnel or post which creates a void in the investment casting, near thetop center of the mold. This reduces the thickness of investmentmaterial around the upper portion of the casing, reducing the tendencyof the investment to deform the wax forms which are also spaced aroundthe casing. This is because a uniform wall thickness of investmentmaterial is created on both inner and outer sides of the wax forms.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich an embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the present invention including elastomerbase, casing and cover, as well as plastic or wax sprue and wax forms;

FIG. 2 is a top plan view of the mold of the present invention;

FIG. 3 is a top plan view of the base of the present invention;

FIG. 4 is a bottom plan view of the base of the present invention;

FIG. 5 is a top plan view of the spoked sprue with ring of the presentinvention;

FIGS. 6 and 7 are sectional views of the spoke and ring respectively,taken along lines 6--6 and 7--7 of FIGS. 6 and 7 respectively;

FIG. 8 is a side elevational view of the sprue of the present invention;

FIG. 9 is a bottom plan view of the sprue of the invention; and

FIG. 10 is a cross-sectional view of the investment casting withembedded wax sprue and forms, before they are burned out of theinvestment, in preparation for shooting molten metal for making aprecision casting of the same shape.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention pertains to the production of dental molds usinginvestment casting vessels.

Referring to the drawings in particular, the invention embodied in FIG.1 comprises a mold and sprue combination for use in investment castingswhich in turn are used for making precision metal castings. Theinvention comprises a casing 12 having open upper and lower ends, a base14 which is fictionally engaged to the lower end of the casing, and acover 16 fictionally engaged with upper end of the casing. The casing,base and cover are all preferably made of the same elastomeric materialsuch as PVC and are advantageously transparent.

A spoked sprue generally designated 18 is attached to base 14 andextends in the volume formed by the vessel comprising the casing, baseand cover. A plurality of wax forms 20 are arranged on the upper surfaceof the sprue. The sprue 18 and forms 20 can be made of the same ordifferent wax or plastic material which is capable of being "burned out"of the investment casting as part of the lost wax casting technique.

Casing 12 has a substantially cylindrical side wall 22 with an outwardlyextending stiffening rim 24 extending around its upper open end, and arelatively thin wall cylindrical collar 26 around its lower open end.Collar 26 has an outer surface which is co-extensive with the outersurface of wall 22, and an inner surface 28 bounded at its upper end byan inwardly extending shoulder or bevel 27 and at its lower end by anoutwardly extending bevel 29.

A plurality of vertically spaced embossed graduation markings 30 areprovided on the inner surface of cylindrical wall 22. Each markingincludes a horizontal mark and a numeric value. The term embossed ismeant to include both raised markings 30 and indented markings 30. Ifraised markings are used, they will be transferred in the form ofindented markings in the cured and hardened investment casting. Ifindented markings 30 are used, they will be transformed into raisedmarkings on the outer surface of the investment casting. For the presentinvention, raised markings 30 on casting 12 are preferred, sinceindented markings on the outer surface of the brittle investment castingare preferred.

Base 14 includes a thick cylindrical neck 32 having a stepped outersurface which defines a small diameter cylindrical surface 34 bounded atthe top and bottom by respective inwardly and outwardly extending bevels33 and 35. Bevels 27, 29 and inner surface 28 of the collar 26fictionally engage the bevels 33, 35 and outer surface 34 of base 14 toseal the casing to the base. The lower flat surface of casing 12 engagesagainst a ring shaped platform surface 36 which is formed below bevel 35in base 14. The outer large diameter surface 37 of base 14 has the samediameter as the outer surface of wall 22. A lower stiffening rim 38extends radially outwardly at the lower end of base 14 and complimentsthe upper rim 24 to reinforce the circular shape of the mold in itsassembled condition, and help separate the mold parts when held in thehands of a lab technician.

It is noted that the generally axially extending surfaces of the mold,such as the surfaces 28, 34, 37 and inner and outer surfaces of wall 22,are generally cylindrical, but in fact, they have a slight taper of 1 or2%, as needed to extract the mold parts from equipment for making themold parts.

Neck 32 includes an upper tapering or conical surface 40. A raisedcircular sighting bead 42 is provided on surface 40 intermediate theinner and outer surfaces of neck 32. The sighting bead may be closer tothe outer surface or closer to the inner surface as desired. The purposeof sighting bead 42 is to provide a visual aide for the laboratorytechnician in placing the wax forms 20. The lab technician must insurethat all wax forms 20 are placed within an imaginary cylindrical surfaceextending vertically from bead 42 and shown by the phantom line 43. Thisinsures that in the final investment casting, a minimum wall thicknessof investment material exists around the wax forms. The sighting bead 42is used after the sprue 18 has been seated in the base 14 as will bedescribed.

Base 14 also includes a central conical hub 44 which forms a funnel inthe end of the investment casting which will communicate with the gatesand channels formed by the sprue 18 and forms 20.

Hub 44 includes a tapered base 45 and a conical post receiver 46 havinga central generally cylindrical blind hole or bore 47 therein.

A ring shaped groove 48 is defined between hub 44 and neck 32. Thecombination of the hub 44 and thick neck 32 reduces the circumfrencialwall thickness of the funnel portion of the investment casting (seeinvestment casting funnel wall 72 in FIG. 10).

A floor surface 50 extends radially between the hub and neck, formingthe bottom inside surface of the base.

As best shown in FIG. 3, an embossed direction marking 52,advantageously in the form of an arrow, is provided at a selectedlocation around the floor surface 50. Here again, the term embossed isused to designate both a raised or an indented marking but the raisedmarking is preferred to form an indented corresponding marking in thetop surface of the funnel wall of the investment casting. This directionmarking is valuable to the laboratory technician, who has positioned thewax forms 20 while viewing the marking 52 before the investment castinghas been formed and who is given an indication of the placement andposition of the wax forms in the investment casting after it hashardened, even though the forms are completely invisible within the bodyof the casting.

As shown in FIGS. 1 and 8, the spoked sprue 18 has a taper post 54 witha beveled lower end at the large diameter of the taper and a smaller endat the top, from which three radially extending spokes 56 extendoutwardly and upwardly from the post. The large diameter lower end ofpost 54 and the direction of its taper helps firmly seal the wax or softplastic post in the blind bore 47 of the base. A sprue ring 58 isconnected to the outer ends of the spokes 56. Referring now to FIGS.5-9, FIG. 8 shows an angle A between the outer surface of tapered post54 and the axis of sprue 18.

As shown at FIG. 5, the 3 spokes 56 are spaced at equal distances aroundthe center of the sprue. Although three spokes have been found to beoptimum for creating the gate channels for the passage of molten metalto the voids created by the wax forms 20, the invention is intended toinclude two or more spokes spaced by equal amounts around the center ofthe sprue and more advantageously, three or more spokes. Two spokes didnot consistently fill the top ring and mounted parts on the ring. Threespokes gave consistent filling. Four spokes also filled consistently butconsumed more alloy without any greater advantage. Also three spokesalways deliver alloy uniformly to the ring, no matter what position theinvesting casting is placed in the machine.

Another important advantage of the invention is the fact that each ofthe spokes 56 and the ring 58 are substantially triangular incross-section as shown in FIGS. 6 and 7. Although the bottom apex of thespokes and ring is advantageously rounded, the top surface is flat. Thiscreates a gate system in the lost wax process which is likewisetriangular in cross section, which has been found to increase thefilling rate of the system with molten metal, and also savesapproximately 40% of the metal in the ultimate precision casting whichis otherwise wasted in the sprue systems having round cross sections.Another advantage to the flat topped configuration of the ring 58 isthat it facilitates arrangement and mounting of the wax forms 20 on theflat surface. The lab technician need not have the difficulty ofbalancing wax forms on a curved upper surface, but rather is providedwith a flat convenient surface to work on. This is further enhanced bythe sighting bead 42.

Returning to FIG. 1, with the sprue 18 engaged to the base 14 (notshown) and the casing 12 and cover 16 engaged to each other to form theclosed vessel (not shown), the graduation markings 50 are used to showthe maximum vertical height of the highest wax form 20 in the vessel.This measurement is noted by the laboratory technician. In the processof pouring the liquid investment solution into the mold vessel, the labtechnician fills the vessel to a level of about 10 mm or greater abovethe highest wax form. Keeping In mind that the vertical graduationsremain as indentations on the outer surface of the investment casting.Referring to FIG. 10, after the casting shown at 60 in FIG. 10 has curedand hardened and is removed from the mold, the laboratory technician,knowing the internal position of the sprue 18 (which would form aprecision casting in the investment) will select a locationapproximately 5 millimeters above the highest wax form (which waspreviously noted). For example, if the highest wax form is at 40 mm,this position is known and shown by the lower dashed lines in FIG. 10. Alevel 5 mm higher (that is, at the 45 mm position) is selected and thencan be used to grind away the top portion of the investment casting.This grinding operation in no way endangers the precision casting as amargin of 5 mm is maintained. At the same time, excess weight andunneeded investment material is removed. This is important because toomuch (greater than 5 mm) of investment over the patterns causes theinvestment to retain too much heat after the molten alloy is cast,retrading the solidification of the cast metal causing porosity. Thedirection marking 62 which was left by the embossed direction marking 52is also available to help the laboratory technician orient theinvestment casting in an appropriate manner since the technician alreadyknows the position and orientation of the casting within the casting 60,from marking 62.

Referring now to FIGS. 1 and 2, cover 16 comprises a disk shaped body 64having an outwardly extending cover rim 66. Body 64 has an outerdiameter that matches the inner diameter of wall 22 and rim 66 fits overthe top of stiffening rim 24 of body 22. A curved lip 65 extendsvertically from the body and rim and forms a curved recess 63 best shownin FIG. 2. The recess 63 is deep enough so that it exposes a small areaof the open top of the body 22, at a hole 25.

In use, the lip 65 and hole 25 are used to facilitate pouring of liquidinvestment into the mold in a manner disclosed in U.S. Pat. No.5,469,908.

As noted above, the graduation markings 30 can be used during thefilling operation, and with the mold held in an upright position, toprovide only as much investment into the vessel as is required toestablish a good margin of material above the highest wax form 20.

In order to avoid waste of investment material and even moreimportantly, in order to establish a substantially uniform thickness ofinvestment material on both sides of the ring shaped pattern of waxforms 20, a tapered plug 68 extends from the center of the lower surfaceof cover body 64.

As shown in FIG. 10, this creates and upper blind funnel 74. Anexamination of FIG. 10 will show that there is substantially equalthickness of investment materials on both sides of the wax forms 20.This avoids distortion that would be caused while the investment curesand expands. Curing of the investment is known to involve expansion ofthe investment material that would cause undesirable constriction anddeformation of the form.

Uniformity of investment around the patterns and subsequently the castmetal is also important for uniform expansion (thermal) during heatingof the casting investment as well as uniform cooling of the investmentsafter metal injection, preventing casting mold distortion.

The use of plug 68 as an integral part of cover 16 automatically centersthe plug in the ring shaped configuration and thus simplified the use ofthe invention over U.S. Pat. No. 4,962,909. U.S. Pat. No. 4,962,909 alsoprovides a blind funnel at the top of the casting but does so in adifferent manner which requires careful alignment and adjustment for asuccessful casting operation.

As shown in FIG. 1, voids 69 are provided inside the hub and neck ofbase 14 to reduce the amount of material needed to form the base 14.While this results in a hollow neck and hub arrangement, this does notadversely affect the invention in any way. In likewise fashion, acentral recess 67 is formed at the top of cover body 64, which recessextends into the center of plug 68. This produces a hollow plug and alighter weight cover, again without reducing the effectiveness of thepresent invention.

As shown in FIG. 4, radial ribs 59 are provided to span the void 69 inthe neck to stiffen the neck. Although four ribs 59 are shown in FIG. 4,a larger or smaller number of ribs may be used depending on the size ofthe mold.

According to the present invention, in particular by using the thickwalled neck in the base and the blind funnel in the cover, an investmentcasting which would normally use 300 grams of investment solution onlyneeds 180 grams according to the present invention. A larger investmentwhich would normally use 500 grams only requires 220 grams. Thisrepresents savings of $2-$3.00 per investment casting and also reducesdistortion in the precision casting. By using triangular spokes and ringin the sprue 18, approximately 40% of the metal in the gate system issaved. This can be on average, a savings of over $50.00 per casting ingold alloy.

It is noted that the alloy or metal is usually 30% to 85% gold, withplatinum or palladium. The cost is about $300 to $400 per troy ounce. Anaverage casting can consume 0.75 to 1.5 ounces of alloy.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A mold assembly for dental investment casting,comprising:a round casing made of resilient plastic material, havingopposite open ends, the casing having an inner surface defining a casingvolume and a lower collar separated from the inner surface by ashoulder; a round base made of resilient plastic material and includinga neck having an outer surface extending along the axis of the casingfor frictional engagement with the collar, the base including a hubextending along the axis, a floor portion extending radially between thehub and the neck, the neck including an upper ring shaped surface with acircular sighting bead on said ring shaped surface for defining animaginary cylindrical surface in said casing when said base and casingare fictionally engaged with each other; and a sprue having a postengaged with said hub and a ring connected to said post and extendinginto said casing when said casing and said base are fictionally engagedwith each other, said ring being within said imaginary cylindricalsurface.
 2. A mold assembly according to claim 1, wherein the hub isconical and includes a central blind top bore therein, said post of saidsprue being tapered with a lower large diameter end and an upper smalldiameter end, the small diameter end being connected to the ring.
 3. Amold assembly according to claim 2, including a plurality of radiallyand upwardly extending spokes, equally spaced around said post andconnected to said ring.
 4. A mold assembly according to claim 3, whereinsaid ring and each of said spokes has a substantially triangularcross-section with a flat top surface and a lower apex.
 5. A moldassembly according to claim 4, wherein the sprue comprises three spokesconnected between the post and said ring.
 6. A mold assembly accordingto claim 1, including a cover made of resilient plastic material, thecover having a disk shaped body for frictional engagement into an upperone of the opposite open ends of the casing, a cover rim extendingoutwardly of the body for covering the casing and a plug extendingdownwardly from a center of a lower surface of said body, into thecasing when the cover, base and casing are fictionally engaged with eachother, the plug being positioned centrally with respect to the spruering.
 7. A mold assembly according to claim 6, including a curved lipupstanding from an upper surface of said body and a recess within saidlip and at a periphery of said body and cover rim for defining anopening in the upper open end of the casing when the cover and casingare fictionally engaged with each other.
 8. A mold assembly according toclaim 7, including a plurality of radially and upwardly extendingspokes, equally spaced around said post and connected to said ring.
 9. Amold assembly according to claim 8, wherein said ring and each of saidspokes has a substantially triangular cross-section with a flat topsurface and a lower apex.
 10. A mold assembly according to claim 9,wherein the sprue comprises three spokes connected between the post andsaid ring.
 11. A mold assembly according to claim 10, including a covermade of resilient plastic material, the cover having a disk shaped bodyfor frictional engagement into an upper one of the opposite open ends ofthe casing, a cover rim extending outwardly of the body for covering thecasing and a plug extending downwardly from a center of a lower surfaceof said body, into the casing when the cover, base and casing arefictionally with respect to the sprue ring.
 12. A mold assemblyaccording to claim 1, including a plurality of vertically spaced andembossed graduation markings on an inner surface of said casing.
 13. Amold assembly according to claim 12, including an embossed directionmarking on said floor portion of said base.
 14. A mold assemblyaccording to claim 1, wherein said neck comprises a thick walled ringaround said hub, said hub and neck extending upwardly from said floorportion for defining a thin walled funnel in an investment castingformed with said mold assembly.
 15. A mold assembly according to claim14, wherein said casing includes a casing rim extending outwardly froman upper open end of said casing, said base including a base rimextending outwardly from a lower end of said base.
 16. A mold assemblyaccording to claim 15, including a cover made of resilient plasticmaterial, the cover having a disk shaped body for frictional engagementinto an upper one of the opposite open ends of the casing, a cover rimextending outwardly of the body for covering the casing and a plugextending downwardly from a center of a lower surface of said body intothe casing when the cover and casing are fictionally engaged with eachother, the plug being positioned centrally with respect to the spruering, said cover rim, base rim and casing rim being substantially equalin outer diameter.
 17. A mold assembly according to claim 16, whereinsaid hub, neck, body and plug are hollow.